Xiaomi Mi Piston Air Capsule Earphones. Excellent sound quality begins with the diaphragm, which is embedded at the heart of any headphone. In order to deliver a superior sound in the mid and bass ranges, we sandwiched an aerospace-grade metal composite layer inbetween two PET layers. This preserves the outstanding treble responses of metal, while improving richness in the lower ranges thanks to the incredible flexibility of PET. This breakthrough design has revolutionized Mi In-Ear Headphones, providing balanced output and superior definition across the entire frequency range.